Add check of the arity of the mocked method.

README.md

@@ -28,11 +28,10 @@ class MyOtherClassTest < MiniTest::Unit::TestCase
 end
 ```
 
-The only real difference of using `MiniTest::FireMock` instead of `MiniTest::Mock` is that if `MyClass` is defined, and `my_method` isn't there, it'll raise a `MockExpectationError`.
+The only real difference of using `MiniTest::FireMock` instead of `MiniTest::Mock` is that if `MyClass` is defined, and `my_method` isn't there, it'll raise a `MockExpectationError`. It checks also for the arity of the method, so it'll raise a `MockExpectationError` if the real method have a different arity than the expectation.
 
 TODO
 ----
 
-- Check for the arity of the methods if they are defined.
 - Mock class/module methods too.
 - Make it work with method_missing (as of now it doesn't, even if the #responds_to? is correct)

lib/minitest/fire_mock.rb

@@ -14,6 +14,10 @@ def expect(name, retval, args = [])
       raise MockExpectationError, "expected #{@constant_name} to define `#{name}`, but it doesn't"
     end
 
+    if method and method.arity != args.size
+      raise MockExpectationError, "`#{name}` expects #{method.arity} arguments, given #{args.size}"
+    end
+
     super(name, retval, args)
   end
 

test/unit/firemock_test.rb

@@ -43,4 +43,11 @@ def test_mock_with_namespace_is_invalid_when_defined_but_dont_responds_to_method
       mock.expect(:not_defined_method, 42)
     end
   end
+
+  def test_valid_mock_with_different_arity
+    mock = MiniTest::FireMock.new('DefinedConstant')
+    assert_raises MockExpectationError, "`defined_method` expects 0 arguments, given 3" do
+      mock.expect(:defined_method, 42, [1,2,3])
+    end
+  end
 end